From all indications there simply is no way currently to do this, or at least no obvious and well documented way that I can find out.
I've invested substantially in Roku for my own private solution, because its pervasive and generally easy to develop for. I have friends and relatives who even have Roku TV sets and it makes it easy to share my channel with them. But besides the under-powered hardware vs the competition and limited format/codec support, what I find truly frustrating is that for such a basic simple feature like directly designating the audio stream, preferably by both ordinal or language code in the API or a functioning audio track selection screen, that we go years and years with no API support and pretty much all questions about this specific topic go ignored. :x Doubly so because the device and roVideoScreen software component are demonstrably able to play the audio stream, there simply is no easy way for me to tell it to do so, either by API or on-screen controls. The screen that pops up saying "Audio selection not available" remains an undocumented mystery.